Quoting Crystal Marie S.:" It's a Grey area. YouTube has deleted or blocked a lot of videos that have copyright infringement issues, so it's generally okay to download them, but they don't catch everything."
Quoting Glo♥:" So would it be ok to download like a popular music video that was posted by their record company or them?"Yeah that's fine.
Quoting Pusherman:" The music industry makes money on videos uploaded by them due to advertising on youtube. If you download ... [snip!] ... view money. It's technically piracy since you didn't pay for exclusive listening rights, and you're not making them ad money."